Raman spectroscopic investigation of H2, HD, and D2 physisorption on ropes of single-walled, carbon nanotubes

We have observed the S- and Q-branch Raman spectra of H2, HD, and D2 adsorbed at 85 K and pressures up to 8 atm on single-walled, carbon nanotubes (SWNT). Comparative data for H2 on graphite and C60 were also collected. Frequency-downshifted and upshifted features were observed in the Q-branch spectra of H2 on C60 and SWNT. These shifts are small and are therefore inconsistent with charge transfer. An H2-surface potential with van der Waals and electrostatic terms was developed and used to estimate the shifts in the frequency of the Q(0) transition of H2 adsorbed in two types of sites. These calculations corroborate the experimental findings and indicate physisorption in multiple sites of the SWNT ropes.     

Modeling and simulations of the behavior of glass particles in a rotating drum in heptane and water vapor atmospheres

We have performed noise measurements on suspended ropes of single wall carbon nanotubes (SWNT) between 1 and 300 K for different values of dc current through the ropes. We find that the shot noise is suppressed by more than a factor 100 compared to the full shot noise 2eI. We have also measured an individual SWNT and found a level of noise which is smaller than the minimum expected. Another finding is the very low level of 1/f noise, which is significantly lower than previous observations. We propose two possible interpretations for the strong shot noise reduction: i) Transport within a rope takes place through few nearly ballistic tubes within a rope and possibly involves non integer effective charges with e ^*∼ 0.3e. ii) A substantial fraction of the tubes conduct with a strong reduction of effective charge (by more than a factor 50). Received 25 January 2002 Published online 19 July 2002     

热效应对纳米焊接界面结构的影响

采用分子动力学方法深入研究热效应对纳米焊接界面结构的影响。具体分析了碳管直径和时间的变化对界面结构的影响规律,并详细给出了1530 K下的焊接过程。结果表明,除碳管(5,5)外,碳管(6,6),(7,7),(9,9),(11,11)中纳米线形成的时间分别为13.8, 14.6, 17.5, 19.6 ps。纳米线是由单根或多根Ni原子链组成,碳管(6,6),(7,7),(9,9),(11,11)中Ni原子链数分别为1,3,7和16。界面结构包括内部焊接和外部焊接。内部焊接的临界直径由碳管(6,6)决定,其值为0.814 nm。在同一时刻,大直径的碳管可获得更大的外部接触长度。外部接触长度的增长速率随碳管直径的增加而增大,接触长度的最大增长速率可达0.013 nm/ps。最后确定了界面结构形成临界温度,发现对于相同直径碳管,外部焊接的临界温度高于内部焊接的临界温度,临界温度与碳管直径无关。     

Physical and mechanical characterization of nanocomposites with carbon nanotubes functionalized with the matrix polymer

Single wall carbon nanotubes (SWNT) are functionalized with poly(vinyl alcohol) (PVA). The functionalized nanotubes are homogeneously dispersed in a PVA solution. Nanocomposite films with low SWNT content (0.1 to 1%) are produced by the wet-casting method. X-ray diffraction shows that the PVA crystallinity was not affected by the presence of the SWNT. The improvement in tensile modulus and strength at such low reinforcement loadings is indicative of good interfacial bonding between the functionalized nanotubes and the polymer matrix.     

Magnetoadsorptive Particles Enabling the Centrifugation‐Free,Preparative‐Scale Separation,and Sorting of Single‐Walled Carbon Nanotubes

A novel magnetic composition of the high surface area particles with amide chemical functionality, Sephacryl S‐200, is reported enabling the preparative‐scale (1 L, ≈5–10 mg) separation of metallic and semiconducting single‐walled carbon nanotubes (SWNT) from completely unpurified and uncentrifuged nanotube stocks. Sephacryl S‐200 has previously been utilized in separating semiconducting SWNT (s‐SWNT) on the laboratory scale. Significantly, use of these magnetic derivative particles in absorptive separation of SWNT allows the unprecedented and industrially scalable purification of both metallic SWNT (m‐SWNT) as well as s‐SWNT directly from uncentrifuged, ultrasonicated surfactant‐based SWNT solutions by simple and scalable magnetic separation. These particles also allowed for the systematic study on the effect of SWNT–polymer interaction time on the resulting SWNT “payloads.” Ultimately, high ‐purity m‐SWNT and s‐SWNT products are independently achieved by controlling the SWNT–polymer interaction time and relative concentrations, as well as SWNT sonication conditions. Furthermore, by controlling these factors, single‐chirality (6,5) s‐SWNT can be isolated with 92% purity directly from unpurified stocks. Thermogravimetric analysis indicates a total process SWNT yield of 1.2% and 1.7% for m‐SWNT and s‐SWNT, respectively. These results demonstrate the potential for a preparative method for separating carbon nanotubes based on electronic properties.     

Four-wave mixing spectroscopy of aqueous suspensions of single-wall carbon nanotubes in the ranges of 0.1–10 and 100–250 cm<Superscript>−1</Superscript>

Distinctive optical properties of single-wall carbon nanotubes (SWNT) are highly sensitive to variations in the environment. Here, we have studied SWNT in aqueous suspensions at a low (less than 0.1 μg ml⁻¹) concentration by four-wave mixing (FWM) spectroscopy in the spectral bands of 0.1 to 10 cm⁻¹ (≈300 GHz) and 100 to 250 cm⁻¹ (3 to 7.5 THz). We directly investigated the hydration layers around SWNT. A comparison of the FWM spectra of an SWNT aqueous suspension and Milli-Q water shows a considerable increase in the intensity of low-frequency Raman modes, which are attributed to the rotational transitions of H₂O₂ and H₂O molecules. We explain the observed phenomenon by the hydrogen peroxide production and formation of a low-density depletion layer at the water-nanotube interface. We have observed several SWNT radial breathing modes ω _RBM =118.5, 164.7, and 233.5 cm⁻¹ in an SWNT aqueous suspension and estimated the corresponding SWNT diameters as ≈2.0, 1.5, and 1 nm.     

Well-defined single-walled carbon nanotube fibers as quantum wires: Ballistic conduction over micrometer-length scales

We report quasi-ballistic conduction in single-walled carbon nanotube (SWNT) fibers at room temperature, with a measured mean free path of 1.0–2.6 μm. The dynamic submersion of vertical SWNT fibers into liquid mercury (Hg) electrode shows plateaus and steps indicative of quasi-ballistic electron transport. This response is described with a newly developed network model that uses surface impurities to simplify the parallel conducting channels. The quasi-ballistic SWNT fibers exhibit a resistance per unit length of 2.5–6.5 kΩ/μm and a mean free path exceeding 1 μm, a length longer than typical via dimensions in existing Si-chip technologies. These results highlight that SWNT fiber conductivity can be enhanced by increasing the metallic to semiconducting SWNT ratio and reducing the surface impurities.     

Carbon nanotubes adhesion and nanomechanical behavior from peeling force spectroscopy

Applications based on single walled carbon nanotube (SWNT) are good example of the great need to continuously develop metrology methods in the field of nanotechnology. Contact and interface properties are key parameters that determine the efficiency of SWNT functionalized nanomaterials and nanodevices. In this work we have taken advantage of a good control of the SWNT growth processes at an atomic force microscope (AFM) tip apex and the use of a low noise (10⁻¹³ m/√Hz) AFM to investigate the mechanical behavior of a SWNT touching a surface. By simultaneously recording static and dynamic properties of SWNT, we show that the contact corresponds to a peeling geometry, and extract quantities such as adhesion energy per unit length, curvature and bending rigidity of the nanotube. A complete picture of the local shape of the SWNT and its mechanical behavior is provided.     

Gas-dynamic consideration of the laser evaporation synthesis of single-wall carbon nanotubes

The abundance of carbon single-wall nanotubes (SWNTs) in soot synthesized by pulsed laser evaporation of graphite is studied over a wide range of synthesis conditions. The derived SWNT growth time-scale appears to be much longer than any characteristic time-scale in a simplified model of the relaxation of a high-pressure hot condensing gas bubble in a background atmosphere. It is concluded that SWNT nucleation and growth take place in relaxed, condensed, thermalized evaporation products at an optimal temperature between 850-1250 °C at a rate of few 7m length per second, which is consistent with a condensed state "precipitation" mechanism for the SWNT formation.     

基于碳纳米管分子封装的一维电子波函数扫描

利用第一性原理,设计并研究了一类基于单臂碳纳米管的分子封装的分子体系．计算表明,半环葫芦脲类化合物可有效封装碳纳米管,引入微弱的分子间相互作用,对碳纳米管的电子态能级结构分布 仅带来微弱影响．半环葫芦脲分子与碳纳米管在管径方向的一维电子态波函数充分耦合,进而有效改变了一些前沿分子轨道的波函数在管径两头的分布以及相应的电子布居浓度．基于电子输运的模拟,发现半环葫芦脲分子在碳纳米管一维方向滑动时的某个电压下的电导变化可准确反映电子态波函数在相应分子导电通道上的一维分布信息．     

On the iron-catalysed growth of single-walled carbon nanotubes and encapsulated metal particles in the gas phase

The production of single-walled carbon nanotubes (SWNT) by catalytic disproportionation of carbon monoxide on iron particles in the gas phase was studied. The addition of hydrogen to the reactants was found to increase the SWNT yield relative to the competing formation of encapsulated iron particles. Adding acetylene, however, did not result in a significant change of the SWNT growth but gave rise to the formation of unwanted self-pyrolysis products. Iron particles are seen to decorate the SWNT and are shown to be single crystalline with several layers of encapsulation by graphitic carbon. All these observations are discussed with respect to the catalytic growth mechanism of SWNT and encapsulated metal particles from CO and hydrocarbon molecules.     

氧化钨纳米线-单壁碳纳米管复合型气敏元件的室温NO2敏感性能与机理

利用溶剂热法合成了一维的氧化钨纳米线, 通过掺入适量单壁碳纳米管(SWNT)制备了基于氧化钨纳米线-SWNT 复合结构的室温气敏元件并评价了其对NO₂气体的室温敏感性能. 利用X射线与扫描电子显微镜表征了材料的微结构, 结果表明, 合成的氧化钨纳米线具有单斜的W₁₈O₄₉结构, 复合材料中SWNT被包埋在氧化钨纳米线中间. 气敏性能测试结果表明, 氧化钨纳米线-SWNT复合结构气敏元件在室温下对NO₂气体表现出了高的灵敏度和超快的响应特性; 较低的SWNT掺入量对获得好的气敏性能有利. 分析了基于复合结构材料气敏元件的可能的气敏机理, 认为元件良好的室温敏感性能与SWNT掺入在复合结构材料中引入大量的贯穿气孔和p-n异质结有关.     

Polarization dependence of the optical absorption of single-walled carbon nanotubes

Anisotropic optical absorption properties of single-walled carbon nanotubes (SWNTs) are determined from a vertically aligned SWNT film for 0.5-6 eV. Absorption peaks at 4.5 and 5.25 eV are found to exhibit remarkable polarization dependence and have relevance to optical properties of graphite. A method for determining a nematic order parameter for an aligned SWNT film based on the collinear absorption peak at 4.5 eV is presented, followed by the determination of the optical absorption cross section.     

Optical anisotropy in single-walled carbon nanotubes

Optical anistropy at optical communication wavelength was observed in films of vertically aligned single-walled carbon nanotubes (SWNTs). We report the control of both the polarization state and transmission of incoming light at 1550 nm by azimuthal and axial tilting of SWNT film about its aligned axis. The experiments reveal that the polarization state of light is susceptible to the azimuthal angle of the aligned direction of a SWNT having semiconductor characteristics and the intensity of the output beam after SWNT film shows cosine function dependence on the axial tilting angle.     

A simple method for creating nanotube field emitters from a surfactant dispersion

A new method has been developed for depositing aligned ropes of single-walled nanotubes (SWNTs) such that a significant number of ropes protrude normally from the surface. The resulting surface has excellent field emission characteristics owing to the high local fields generated at the tips of the protruding nanotubes. Further development of the technique may lead to the construction of a reasonably homogeneous surface bristling with nanotube ropes, which could allow spatially resolved neutral atom detection via field ionization.     

温控电弧放电法大量制备单壁碳纳米管

采用可以控制真空室温度的改进型直流电弧炉，在氮气和氦气(1∶1)混合气氛下，使用Co-Ni(1∶1wt%)合金催化剂，通过控制温度等工艺条件，在容器内壁生成了大量单壁碳纳米管，尤其在阴极与阳极之间有大量的宏观网状薄膜.通过两步纯化方法：在500℃空气中烘烧30min；再用37%盐酸浸泡72h，用去离子水过滤至中性烘干.经SEM，HRTEM，XRD，Raman观察分析，纯化后其纯度高(>95％)、管径均匀(1.24—1.38nm).实验结果表明：温度强烈影响单壁碳纳米管的产量，不同温度下管子的纯度、产量都有差异，在温度为600℃时，其纯度达到70％，产量为12g/h. 关键词： 单壁碳纳米管 电弧法 温度影响     

X-ray irradiation effect of double walled carbon nanotube

We have studied double-walled carbon nanotube (DWNT) irradiated by soft X-ray by Raman scattering spectroscopy and the spectral characteristics are compared to single-walled carbon nanotube (SWNT) irradiated under the same condition. We proved that DWNT is more stable for the X-ray induced defect formation than SWNT. Moreover, we found that the outer tube of DWNT was more sensitive on X-ray irradiation than the inner tube. The defect was recovered by annealing in Ar at lower temperature than that of SWNT. Based on these results, we inferred that X-ray irradiation leads to formation of interstitial-vacancy pairs, Frenkel defects, in carbon nanotube. The interstitial-vacancy separation on the inner tube of DWNT is conceivably shorter than that of the outer tube.     

Molecular dynamics simulations of hydrogen adsorption/desorption by palladium decorated single-walled carbon nanotube bundle

Utilising molecular dynamics simulations, the hydrogen molecules adsorption isotherms of the (8,?0) palladium decorated single-walled carbon nanotube (SWNT) were obtained. The hydrogen adsorption was studied on the external, interstial and internal surfaces of the SWNT bundle at several temperatures ranging from 77 to 400?K. The results were compared with the bare single-walled carbon nanotube bundle under the same conditions. The decorated carbon nanotube bundle hydrogen adsorption was significantly higher than that of the bare one. The hydrogen desorption and readsorption were studied using temperature as the readsorption/desorption variable. The rate constants were calculated for the hydrogen desorption at different temperatures. The calculated decorated SWNT bundle hydrogen desorption activation energy was higher than that for the bare SWNT bundle. The calculated activation energies for the hydrogen desorption in both nanotube bundles specified the temperature dependency of hydrogen desorption.     

Influence of chirality on the thermal conductivity of single-walled carbon nanotubes

The influence of chirality on the thermal conductivity of single-walled carbon nanotubes(SWNTs) is discussed in this paper, using a non-equilibrium molecular dynamics(NEMD) method. The tube lengths of the SWNTs studied here are 20, 50, and 100 nm, respectively, and at each length the relationship between chiral angle and thermal conductivity of a SWNT is revealed. We find that if the tube length is relatively short, the influence of chirality on the thermal conductivity of a SWNT is more obvious and that a SWNT with a larger chiral angle has a greater thermal conductivity. Moreover, the thermal conductivity of a zigzag SWNT is smaller than that of an armchair one. As the tube length becomes longer, the thermal conductivity increases while the influence of chirality on the thermal conductivity decreases.     

Gases Do not adsorb on the interstitial channels of closed-ended single-walled carbon nanotube bundles

We have experimentally determined the binding energies of Xe, CH4, and Ne on samples of closed-ended single-wall nanotube (SWNT) bundles. We find values for these quantities which are larger by approximately 75% on the SWNT samples than the values found for the same adsorbates on planar graphite. We have also determined the monolayer capacity of a SWNT sample using Xe and Ne adsorption. A comparison of all of our results leads us to conclude that none of the gases studied adsorb on the interstitial channels in the SWNT bundles.